1. Field of the Invention
The present invention relates to optical components for reversing and inverting images, and to methods of manufacturing them accurately, robustly and cost-effectively. More specifically, the present invention relates to the efficient manufacture of a frameless hollow roof mirror constructed from two plane mirrors joined together by optical contact bonding.
2. Description of Related Art
Sometimes optical systems, for example those associated with telescopes and microscopes, must be configured to reverse and/or invert an image of an object.
As is well-known and illustrated in FIG. 1, a plane mirror PM is a simple component for inverting an image I of an object O. As is also well-known and illustrated in FIG. 2, two plane mirrors PM disposed at a right angle form a hollow roof mirror HRM, which is a simple component for both inverting and reversing an image I of an object O.
Unfortunately, it has proven challenging to build hollow roof mirrors HRM accurately and robustly at reasonable cost and weight. Abutting two delicate plane mirrors PM into a roof configuration demands exacting tolerances during manufacture and assembly. Tiny gaps or misalignments tend to deform or destroy the optical path, as can excess of the adhesive commonly used to retain the plane mirrors PM in a roof configuration.
The problem of misaligned elements is also a concern in terms of durability. Over time, or through extended or extreme use, poorly designed elements that were originally aligned might slip out of alignment.
These manufacturing, assembly, and durability issues are important in many precision applications, for example in optical components associated with sensitive measurement instruments such as telescopes and microscopes. The challenges have been particularly present in manufacturing telescopes and telescope accessories because they must be sufficiently robust to survive use in the field. They must survive vibrations during transportation to the field. They must survive temperature changes between storage, transportation, and field environments. They must survive bad weather in the field. And they must resist impact damage from hard use.
As illustrated in FIG. 3, one approach to satisfying these challenges has been to substitute two solid roof prisms SRP for a hollow roof mirror, for example configuring two solid roof prisms SRP into a Porro prism PP. So configured, the solid roof prisms SRPs take advantage of the phenomenon of total internal reflection to function as reflectors, not refractors. Light entering the solid roof prism SRP through a transmission facet TF exits though the same transmission facet TF, after being twice internally reflected by reflection facets RF.
Solid roof prisms SRP and Porro prisms PP have the advantage of being more robust than hollow roof mirrors HRM. The reflection surfaces of the reflection facets RF being within the prism, the prism material is in essence being used as both reflector and frame. However, solid roof prisms SRP also have significant disadvantages, most critically that they are expensive to manufacture, particularly in larger sizes. High-quality glass must be carefully pressure-shaped under extreme heat and then slowly cooled; however, even when great care is taken, this heating/cooling process still has a tendency to cause defects within the prism that can deform or destroy the optical path.
As illustrated in FIG. 4, another approach to satisfying these challenges has been to abut two prisms P together to create a hollow roof between them, by applying a reflective coating to the exterior surface of one facet on each prism (the reflection facets RF) and disposing the two reflection facets RF to form a right angle between them. Such a hollow roof prism HRP has a number of advantages. It is relatively robust and the interior of the component prisms P need not be free of optical defects, because the optical path remains outside the hollow roof prism HRP. However, it also has disadvantages. The component prisms P are still more challenging to manufacture than simple plane mirrors and if they are misaligned during assembly or use, they may not form an optically accurate roof.
As illustrated in FIG. 5, still another approach to satisfying these challenges has been to build a framed hollow roof mirror FHRM, by disposing two plane mirrors PM at a right angle to form a roof and supporting the plane mirrors PM with various frame components, for example a base B, opposing lateral supports LS, a plurality of shock-damping connectors SDC, and opposing end-braces EB, one of which has been hidden from view in FIG. 5 so as not to obscure the other items. While a framed hollow roof mirror FHRM benefits from the advantages that plane mirrors PM provide over prisms, it also suffers from the disadvantages inherent in a framing mechanism, for example additional weight and manufacturing and maintenance complications. Furthermore, it is generally difficult to form a fine junction between the two plane mirrors PM at the apex of the roof and to maintain that junction during use.
Accordingly, what is needed is a better way to manufacture and assemble a robust frameless hollow roof mirror.